The present invention relates to bonded adsorbent structures particularly suited for filtration of gases and vapors and respirators incorporating such bonded adsorbent structures as the active gas and vapor filtration elements. The bonded adsorbent structures are of substantially uniform thickness and density throughout resulting in uniformity of air flow therethrough.
Adsorbent structures have long been used for the filtration of fluids and some forms have been specifically developed for use in gas and vapor respirators. These known structures are generally classified according to the manner in which the adsorbent material is supported and include (a) packed beds, (b) loaded nonwovens, (c) loaded foams and (d) bonded adsorbents.
Of these known adsorbent structures, only the packed beds have been developed sufficiently to meet the stringent filtration and air flow requirements necessary for gas and vapor respirator approval from the pertinent regulatory agency. In such packed beds, adsorbent particles are constrained in a container by compressive forces imposed on and transmitted through the particle bed by rigid grids and screens which cover the inlet and outlet areas. Virtually all such packed bed filters are cylindrical, have constant thickness or bed depth and have a planar inlet and outlet. The adsorbent particles are filled layerwise by pouring through screens. The screens scatter the particles as they fall resulting in a level filled bed packed to substantially maximum density per unit volume. The compressive forces applied by the constraining grids and screens restrain particle movement to thereby minimize flow channeling within the packed bed.
Although gas and vapor respirators with packed bed filter elements satisfy the necessary performance parameters, the very nature of the packed beds has imposed severe constraints on overall respirator design. Thus, their cylindrical geometry dictates incorporating the filter element as an appendage (external cartridge) on the respirator which results in some interference with vision and an increase in the number of respirator parts. Another problem encountered with packed beds for use in a volume sensitive product such as a respirator is that the retaining grids and screens themselves add to the total volume and consequently add bulk to the filter element. A still further problem is experienced when a packed bed respirator is combined in series with a particulate filter for use in environments containing particulates as well as vapor hazards such as in paint spray applications. In this situation, the retaining grids and screens create nonuniform airflow pathways within the particulate filter resulting in reduced utilization of the filter media and increased pressure drop therethrough.
Adsorbent loaded nonwoven structures such as described in U.S. Pat. No. 3,971,373 contain adsorbent particles in the interstices between the fibers forming the nonwoven web. Such structures permit the manufacture of conformable shaped respirators thus overcoming the design restrictions imposed by the geometry of packed bed adsorbent structures. However, the high density of adsorbent particles achieved in the packed bed structures is lost in the adsorbent loaded nonwoven structures because the fibers themselves act as spacers between the adsorbent particles. This low adsorbent density makes it difficult, if not impossible, to achieve the filtration requirements for approvable respirators since it is difficult to pack sufficient adsorbent particles into the small available volume of a respirator. Another form of adsorbent loaded nonwoven structure is adsorbent paper where adsorbent particles are incorporated in the spaces between the paper fibers; these adsorbent papers are also lower density structures.
Open celled loaded foam structures containing adsorbent particles dispersed within and bonded in the foam structure have been developed for various uses, e.g., as an adsorbent composite for evaporative emission control for automobiles (U.S. Pat. No. 3,813,347), a carbon impregnated foam particularly suited for protective clothing against noxious chemicals in liquid or vapor form (U.S. Pat. No. 4,046,939) and an impregnated foam sheet deodorizer insole (U.S. Pat. Re. No. 29,501). Most of the loaded foam structures also suffer the limited density disadvantage of the loaded nonwoven structures thus limiting their use in respirators.
Bonded adsorbent structures have been utilized as liquid filters for many years. While these structures have had the potential for the high adsorbent densities needed for respirators and other critical air filter uses, that potential has not been recognized and exploited.
The known bonded adsorbent structures can be subdivided into two major classifications, viz., those in which the contaminant must first pass through a polymeric binder coating surrounding the adsorbent particle before it is adsorbed by the particle and those where the contaminant encounters the adsorbent particle through uncoated areas on the adsorbent surface.
Examples of bonded adsorbent structures where the adsorbent particles are coated by a polymeric binder are U.S. Pat. No. 3,091,550 directed to semi-rigid materials having a bonded adsorbent coating thereon and U.S. Pat. No. 4,386,947 directed to apparatus for adsorbing fuel vapor in an internal combustion engine wherein the vapor adsorbent is preferably formed from layers of molded monolithic honeycombed activated carbon bodies.
The second type of bonded adsorbent structures, where portions of the adsorbent particle surface are exposed, is exemplified by U.S. Pat. Nos. 3,217,715; 3,353,544; 3,354,886; 3,375,933; 3,474,600; 3,544,507; 3,645,072; 3,721,072; 3,919,369; 4,061,807 and 3,538,020. Of the myriad of intended applications for these bonded adsorbent structures, only U.S. Pat. No. 3,544,507 suggests, in passing, that the agglomerated carbon particles produced could be used as gas mask filters, presumably as a packed bed cartridge. U.S. Pat. No. 3,538,020 is directed to bonded adsorbent bodies comprised of fluid treating aggregate particles such as ion exchange resins, activated charcoal, manganese greensand, sawdust and like materials bound together in closely packed abutting relationship in a matrix of a polymeric material such as polyurethane, the aggregate particles being spaced essentially as they would be in a packed bed. It is, however, expected that a significant portion of the interstitial volume will be occupied by the binder matrix with a resultant increase in pressure drop in the bonded structures. Since no respirator use is suggested in the patent, any possible respirator use to be inferred for such structures would be as substitutes for packed bed filter cartridges. While this patent and a number of the above noted patents state that the bonded adsorbent structures could be molded into any desired shape, most of the shapes exemplified are flat or cylindrical bodies. U.S. Pat. No. 3,721,072 does disclose a differently shaped filter comprising activated carbon granules bonded together into a monolithic extended surface shape in the form of a wave, the filter being particularly useful in air handling systems according to the patentee.
The fact remains that none of the patents specifically addresses respirator applications nor provides any basis for concluding that such bonded adsorbent structures could be used as the filter elements in respirators where high dynamic capacity and high efficiency contaminant removal with low pressure drops and uniform air flow are essential characteristics.